1. Field of the Invention
The present invention relates to a method for preparing cycloaliphatic diamine by hydrogenating aromatic diamine in the presence of a ruthenium catalyst, in which the ruthenium catalyst is pre-treated with air or oxygen at a temperature of 50 to 200.degree. C., without adding a base.
2. Description of the Related Art
A polyurethane is a polymer having a carbamate group (--NHCOO--), and is prepared by reacting diisocyanate and polyol such as ethylene glycol. The properties of the polyurethane depend on its raw materials or its preparation method, and especially, the structure of a diamine which is a starting material of the diisocyanate. Urethane polymers derived from aromatic diisocyanates undergo slow oxidation in the presence of air and light, causing a discoloration which is unacceptable in some applications. However, urethane prepared from cycloaliphatic diamine is stable against light and air, as well as structurally flexible, which has a better properties compared with urethane prepared from an aromatic diamine.
There are substantial literature in the art with respect to the hydrogenation of aromatic amines, especially, methylenedianiline to produce bis(4-aminocyclohexyl)methane, or 1,2-, 1,3-, 1,4-phenylenediamine to produce corresponding cyclohexane diamine. U.S. Pat. No. 2,511,028 and U.S. Pat. No. 2,606,924 disclose a method of preparing bis(4-aminocyclohexyl)methane from methylene dianiline under a pressure of 200-1,000 psig and at a temperature of 80-270.degree. C. in the presence of a noble metal such as ruthenium, rhodium, iridium or a mixture thereof or with a platinum or palladium, either as the hydroxide, oxide, or the metal itself on an insert support.
U.S. Pat. No. 3,636,108, U.S. Pat. No. 3,697,449 disclose a hydrogenation of an aromatic diamine using a ruthenium impregnated upon a support, wherein the activity and efficiency of the catalyst is increased by treating the catalyst and support with an alkali metal hydroxide or alkoxide. According to these disclosures, such alkali moderation of the catalyst could be effected prior to hydrogenation or in situ during the hydrogenation. They also describe that formation of tars, decomposition products and/or condensation products formed during the hydrogenation can be reduced and the catalyst can be used repeatedly without catalyst regeneration.
U.S. Pat. No. 3,591,635 and U.S. Pat. No. 3,856,862 disclose a method for hydrogenating an aromatic diamine by pretreating rhodium with NH.sub.4 OH or ammonia. It has been known that, in the early experiment of the hydrogenation of aniline, ammonia suppressed the formation of by-products, but deactivated the catalyst. It has been reported that similar phenomenon also occurs in the presence of an alkali alkoxide or alkali hydroxide such as LiOH or NaOH.
U.S. Pat. No. 4,448,995 discloses that the hydrogenation reaction should be carried out in an anhydrous state or at least maintained so that water concentration is less than 0.5% by weight to reduce the amount of N-alkylated and higher boiler by-products. In addition, the patent states that lithium salts reduce by-products. However, U.S. Pat. No. 4,946,998 reported that the presence of LiOH increases the production of high molecular weight products.
However, these conventional methods described above are not adequate for industrial processes due to a low yields and the requirement of a corrosive base.
Therefore, the present invention is directed to provide a method for the hydrogenation of aromatic diamine compounds by pre-treating a supported ruthenium catalyst with oxygen or air. By this invention, the hydrogenation reaction time is significantly reduced and the formation of by-products are greatly suppressed.